350-501 Architecture Practice Question
In an IS-IS segment routing network, after the global 'segment-routing mpls' configuration, which IS-IS configuration is mandatory for label allocation?
⚠ Common exam trap
Cisco often tests that candidates confuse the global 'segment-routing mpls' command with the IS-IS sub-mode 'segment-routing mpls' command, leading them to think the global command alone is sufficient for label allocation.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
segment-routing mpls under router isis
The global 'segment-routing mpls' command enables MPLS Segment Routing on the device, but it does not automatically enable label allocation under IS-IS. The mandatory IS-IS configuration to trigger label allocation is 'segment-routing mpls' under the 'router isis' process. Without this sub-mode command, IS-IS will not allocate MPLS labels for prefixes, even if prefix-SIDs are configured on interfaces.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
mpls label protocol sr
Why it's wrong here
This is a global command, not under IS-IS, and is not mandatory if segment-routing is already enabled globally.
- ✓
segment-routing mpls under router isis
Why this is correct
This command is required to enable segment routing label allocation within the IS-IS process.
- ✗
prefix-sid under the loopback interface
Why it's wrong here
Prefix-SID configuration is for assigning specific SIDs to prefixes, but label allocation works even without explicit prefix-SIDs using dynamically allocated labels.
- ✗
is-type level-1-2
Why it's wrong here
IS type is for routing scope and does not enable label allocation for SR.
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